LAUSR

A 36yearold was referred to our centre following excision of a large retroperitoneal mass. This was found to primarily be hyalinevascular subtype Castleman disease (HVCD), but with two nodules of follicular dendritic cell sarcoma (FDCS). FDCS is a rare neoplasm of spindle and ovoid cells with known occasional association with HVCD. Histological features of HVCD from this patient are shown top left with the classic appearance of an atrophic germinal centre and ‘onion skin’ distribution of mantle cells. On computed tomography (CT) imaging prior to excision a large heterogenous enhancing retroperitoneal mass was seen adjacent to the liver and right kidney (top right). An intensely contrastenhancing mass is a typical CT appearance for Castleman disease. Bright nodal enhancement is not a typical imaging feature of the more than 45 currently recognised lymphoma categories, and Castleman disease should always be considered in this setting. On subsequent postresection surveillance imaging a small mass without significant enhancement was identified in the resection bed (bottom left). Given the lack of enhancement it was postulated that this might represent remaining FDCS rather than HVCD. FDCS was confirmed on subsequent histology with malignant spindle cells stained for CD21 shown bottom right. Of note in this case is the importance of considering concomitant FDCS in the setting of Castleman disease, to ensure relevant immunohistochemistry is performed. Identification of FDCS in this context is important as its presence may indicate the need for additional treatment.